The Seoul metropolitan area, the most densely populated part of the Korean Peninsula, features complex subsurface structures and seismogenic faults, though their characteristics remain ambiguous due to low seismicity and limitations in fault investigation. High-resolution velocity models can provide constraints for identifying subsurface faults by detecting elongated low-velocity anomalies along fault zones. Recently, a dense seismic network was deployed in this area, facilitating the use of Helmholtz tomography, an array-based method that accounts for finite-frequency effects. Utilizing Helmholtz tomography, we obtained a high-resolution S-wave velocity model down to a depth of 50 km with waveform data recorded at 74 broadband seismic stations. We found that a linear low-velocity anomaly along the Pocheon fault extends to the uppermost mantle, with an increasing width with depth. In contrast, the Dongducheon fault, which traverses Seoul from north to south, is not well imaged, indicating its current weak activity. Another linear low-velocity anomaly extends southwest through Seoul from northern Seoul, potentially representing the extension of the Pocheon fault based on similar strike and dip directions. Additionally, a large lateral low-velocity anomaly is identified in the lower crust beneath the northern part of the Seoul metropolitan area, interpreted as a ductile décollement, connected with the Pocheon, Wangsukcheon, and possibly Gyeonggang faults. This study successfully identified the extensions and orientations of subsurface faults beneath the Seoul metropolitan area down to the uppermost mantle, which is critical for seismic hazard predictions and earthquake simulations in this highly populated area.

中文翻译：

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亥姆霍兹断层扫描显示朝鲜半岛首尔大都会区的地壳和最上部地幔 S 速度结构


首尔大都会区是朝鲜半岛人口最稠密的地区，具有复杂的地下结构和地震断层，但由于地震活动低和断层调查的限制，它们的特征仍然不明确。高分辨率速度模型可以通过检测沿断层带的细长低速异常来为识别地下断层提供约束。最近，在该区域部署了一个密集的地震网络，促进了亥姆霍兹断层扫描的使用，这是一种基于阵列的方法，可以解释有限频率效应。利用亥姆霍兹断层扫描，我们获得了低至 50 公里深度的高分辨率 S 波速度模型，并在 74 个宽带地震站记录了波形数据。我们发现，沿抱川断层的线性低速异常延伸到最上层的地幔，宽度随着深度的增加而增加。相比之下，从北到南横穿首尔的东豆川断层没有很好地成像，表明它目前的活动较弱。另一个线性低速异常从首尔北部向西南延伸穿过首尔，可能代表了基于相似走向和倾角方向的抱川断层的延伸。此外，在首尔大都会区北部下方的下地壳中发现了一个大型横向低速异常，被解释为韧性的凹陷，与抱川、望淑川和可能的京江断层有关。这项研究成功地确定了首尔大都会区地下断层的延伸和方向，一直延伸到最上部的地幔，这对于这个人口稠密地区的地震灾害预测和地震模拟至关重要。